Hart C. Anthony, Shears Paul. Color Atlas of Medical Microbiology.pdf

General Aspects of Fungal Disease 355

Table 5.2 Continued: Overview of the Most Important Mycoses in Humans

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cific immunity, especially the cellular type. The role of humoral immunity in specific immune defense is secondary.

Diagnosis

The primary concern here is identification of the pathogen.

&Microscopy. Native preparation: briefly heat material under coverslip with 10% KOH. Stained preparation: stain with methylene blue, lactophenol blue, periodic acid-Schiff (PAS), ink, etc.

&Culturing. This is possible on universal and selective mediums. Sabouraud dextrose agar can contain selective agents (e.g., chloramphenicol and cycloheximide), this medium has an acid pH of 5.6. The main identifying structures are morphological, in particular the asexual and, if present, sexual re-

5productive structures. Biochemical tests are used mainly to identify yeasts and are generally not as important in mycology as they are in bacteriology.

&Serology. By the identification of antibodies to special fungal antigens in patient’s serum. The Interpretation of serological findings is quite difficult in fungal infections.

&Antigen detection. By finding of specific antigens in the diagnostic material by direct means using known antibodies, possible in some fungal infections (e.g., cryptococcosis).

&Cutaneous test. Cutaneous (allergy) tests with specific fungal antigens can be useful in diagnosing a number of fungal infections.

&Nucleic acid detection. Combined with amplification, such tests are useful for rapid detection of mycotic diseases in immunocompromised patients.

Therapy

A limited number of anti-infective agents are available for specific treatment of fungal infections:

& Polyenes. These agents bind to membrane sterols and destroy the membrane structure:

—Amphotericin B. Used In systemic mycoses. Fungicidal activity with frequent side effects. There are conventional galenic form and (new) various lipid forms.

—Nystatin, natamycin. Only for topical use in mucosal mycoses.

General Aspects of Fungal Disease 357

& Azoles. These agents disrupt ergosterol biosynthesis. Their effect is mainly fungistatic with possible gastrointestinal side effects. Hepatic functional parameters should be monitored during therapy:

— Ketoconazole. One of the first azoles. No longer used because of side effects.

— Fluconazole. Oral or intravenous application. For the treatment of surface and systemic mycoses and cryptococcal meningitis in AIDS patients.

— Itraconazole. Oral and intravenous application. Use in systemic and cutaneous mycoses and also for the treatment of aspergillosis.

— Voriconazole. Oral and intravenous application. Good activity against

Candida and Aspergillus. No acitivity against Mucorales.

&Allylamines. Terbinafine. By oral and topical application to treat dermatomycoses. Inhibition of ergosterol biosynthesis.

&Echinocandins. Caspofungin has been approved as a salvage therapy in refractory aspergillosis. It is useful also in oropharyngeal and esophageal candidiasis. Inhibition of the biosynthesis of glucan of the cell wall.

&Griseofulvin. This is an older antibiotic used in treatment of dermatomycoses. By oral application, therapy must often be continued for months.

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Primary Mycoses

& Primary systemic mycoses include histoplasmosis (Histoplasma capsulatum), North American blastomycosis (Blastomyces dermatitidis), coccidioidomycosis (Coccidioides immitis), and South American blastomycosis (Paracoccidioides brasiliensis). The natural habitat of these pathogens is the soil. Their spores are inhaled with dust, get into the lungs, and cause a primary pulmonary mycosis. Starting from foci in the lungs, the organisms can then be transported, hematogenously or lymphogenously, to other organs including the skin, where they cause granulomatous, purulent infection foci. Laboratory di-

6agnostics aim at direct detection of the pathogens under the microscope and in cultures as well as identification of antibodies. The therapeutics used to treat these infections are amphotericin B and azoles. All of the primary sys-

temic mycoses are endemic to certain geographic areas, in some cases quite limited in extent. Central Europe is not affected by these diseases. They are not communicable among humans. &

Histoplasma capsulatum (Histoplasmosis)

Histoplasma capsulatum is the pathogen responsible for histoplasmosis, an intracellular mycosis of the reticuloendothelial system. The sexual stage or form of this fungus is called Emmonsiella capsulata.

Morphology and culture. H. capsulatum is a dimorphic fungus. As an infectious pathogen in human tissues it always forms yeast cells (Fig. 6.1). The small individual cells are often localized inside macrophages and have a diameter of 2–3 lm.

Giemsa and gram staining do not “take” on the cell walls of H. capsulatum, for which reason the cells often appear to be surrounded by an empty areola, which was incorrectly taken to be a capsule, resulting in the designation H. capsulatum. This species can be grown on the nutrient mediums normally used for fungal cultures. H. capsulatum grows as a mycelium in two to three weeks on Sabouraud agar at a temperature of 20–30 8C.

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Pathogenesis and clinical picture. The natural habitat of H. capsulatum is the soil. Spores (conidia) are inhaled into the respiratory tract, are taken up by alveolar macrophages, and become yeast cells that reproduce by budding. Small granulomatous inflammatory foci develop. The pathogens can disseminate hematogenously from these primary infection foci. The reticuloendothelial system (RES) is hit particularly hard. Lymphadenopathies develop and the spleen and liver are affected. Over 90% of infections remain clinically silent. The clinical picture depends heavily on any predisposing host factors and the infective dose. A histoplasmosis can also run its course as a respiratory infection only. Disseminated histoplasmoses are also observed in AIDS patients.

Diagnosis. Suitable material for diagnostic analysis is provided by bronchial secretion, urine, or scrapings from infection foci. For microscopic examination, Giemsa or Wright staining is applied and yeast cells are looked for inside the macrophages and polymorphonuclear leukocytes. Cultures on blood or Sabouraud agar must be incubated for several weeks. Antibodies are detected using the complement fixation test and agar gel precipitation. The diagnostic value of positive or negative findings in a histoplasmin scratch test is doubtful.

Therapy. Treatment with amphotericin B is only indicated in severe infections, especially the disseminated form.

Epidemiology and prevention. Histoplasmosis is endemic to the midwestern USA, Central and South America, Indonesia, and Africa. With few exceptions,

360 6 Fungi as Human Pathogens

Western Europe is free of the disease. The pathogen is not communicable among humans. No special prophylactic measures are taken.

Coccidioides immitis (Coccidioidomycosis)

Morphology and culture. C. immitis is an atypical dimorphic fungus. In cultures, this fungus always grows in the mycelial form; in body tissues, however, it neither buds nor produces mycelia. What is found in vivo are spherical structures (spherules) with thick walls and a diameter of 15–60 lm, each filled with up to 100 spherical-to-oval endospores.

C. immitis is readily cultivated on the usual fungus nutrient mediums. After five days of incubation, a white, wooly (fuzzy) mycelial colony is observed. One of the morphological characteristics of the mycelium is the asexual arthrospores seen as separate entities among the hyphae.

Pathogenesis and clinical picture. The infection results from inhalation of dust containing arthrospores. Primary coccidioidomycosis is always localized in the lungs, whereby the level of manifestation varies from silent infections

6(60% of infected persons) to severe pneumonia. Five percent of those infected develop a chronic cavernous lung condition. In fewer than 1%, hematogenous dissemination produces granulomatous lesions in skin, bones, joints, and meninges.

Diagnosis. The available tools are pathogen detection in sputum, pus, cerebrospinal fluid or biopsies, and antibody identification. The spherules can be seen under the microscope in fresh material. The fungus can be readily cultured on Sabouraud agar at 25 8C. The resulting arthrospores are highly infectious and must be handled very carefully. Antibodies can be detected using the complement fixation test, gel precipitation or latex agglutination. A coccidioidin skin test measuring any cellular allergy to components of the fungus is used as an initial orientation test if an infection is suspected.

Therapy. Amphotericin B can be used to treat the disseminated forms. An oral azole derivative will serve as an alternative, or for use, in clinically less severe forms.

Epidemiology and prevention. Coccidioidomycosis is endemic to desert areas of California, Arizona, Texas, New Mexico, and Utah and is only rarely observed elsewhere. The source of infection is the fungus-rich soil. Animals can also be infected. This disease is not transmitted among humans or from animals to humans.

Primary Mycoses 361

Blastomyces dermatitidis (North American Blastomycosis)

Blastomyces dermatitidis is a dimorphic fungus that causes a chronic granulomatous infection. The pathogens occur naturally in the soil and are transmitted to humans by inhalation.

The primary blastomycosis infection is pulmonary. Secondary hematogenous spread can lead to involvement of other organs including the skin. Laboratory diagnostic methods include microscopy and culturing to identify the fungus in sputum, skin lesion pus, or biopsy material. Antibody detection using the complement fixation test or agar gel precipitation is of limited diagnostic value. Amphotericin B is the therapeutic agent of choice. Untreated blastomycoses almost always have a lethal outcome.

Blastomycosis occurs mainly in the Mississippi Valley as well as in the eastern and northern USA. Infections are also relatively frequent in animals, especially dogs. Susceptible persons cannot, however, be infected by infected animals or humans. There are no prophylactic measures.

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Opportunistic Mycoses (OM)

& Opportunistic mycoses (OM) that affect skin and mucosa as well as internal organs are caused by both yeast and molds. A precondition for development of such infections is a pronounced weakness in the host’s immune defenses. Candidiasis is an endogenous infection. Other OMs are exogenous infections caused by fungi that naturally inhabit the soil or plants. These environmental fungi usually invade via the respiratory tract. The most important are aspergillosis, cryptococcosis, and the mucormycoses. Besides Candida and other yeasts, phaeohyphomycetes and hyalohyphomycetes, which are only very mildly pathogenic, can also cause systemic infections. All OMs have a primary infection focus, usually in the upper or lower respiratory tract. From this focus, the pathogens can disseminate hematogenously and/or lymphogenously to infect additional organs. Infection foci should be removed surgically if feasible. Antimycotic agents are used in chemotherapy. In infected immunocompromised patients, the prognosis is usually poor. &

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Candida (Soor)

At least 70% of all human Candida infections are caused by C. albicans, the rest by C. parapsilosis, C. tropicalis, C. guillermondii, C. kruzei, and a few other rare

Candida species.

Candida albicans

Fig. 6.2 Candida albicans. a Morphological forms.

b Gram staining of sputum: Gram-positive yeast cells and hyphae. Clinical diagnosis: candidiasis of the respiratory tract.

Opportunistic Mycoses (OM) 363

Morphology and culture. Gram staining of primary preparations reveals C. albicans to be a Gram-positive, budding, oval yeast with a diameter of approximately 5 lm. Gram-positive pseudohyphae are observed frequently and septate mycelia occasionally (Fig. 6.2).

C. albicans can be grown on the usual culture mediums. After 48 hours of incubation on agar mediums, round, whitish, somewhat rough-surfaced colonies form. They are differentiated from other yeasts based on morphological and biochemical characteristics.

Pathogenesis and clinical pictures. Candida is a normal inhabitant of human and animal mucosa (commensal). Candida infections must therefore be considered endogenous. Candodoses usually develop in persons whose immunity is compromised, most frequently in the presence of disturbed cellular immunity. The mucosa are affected most often, less frequently the outer skin and inner organs (deep candidiasis). In oral cavity infections, a white, stubbornly adherent coating is seen on the cheek mucosa and tongue. Patho-

Clinical Forms of Candidosis

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a

Fig. 6.3 a Oral soor; surface infection of cheek mucosa and tongue by Candida albicans in an AIDS patient.

b Chronic mucocutaneous candidiasis in a child with a cellular immunodeficiency syndrome.

b

364 6 Fungi as Human Pathogens

morphologically similar to oral soor is vulvovaginitis. Diabetes, pregnancy, progesterone therapy, and intensive antibiotic treatment that eliminate the normal bacterial flora are among the predisposing factors. Skin is mainly infected on the moist, warm parts of the body. Candida can spread to cause secondary infections of the lungs, kidneys, and other organs. Candidial endocarditis and endophthalmitis are observed in drug addicts. Chronic mucocutaneous candidiasis is observed as a sequel to damage of the cellular immune system (Fig. 6.3).

Diagnosis. This involves microscopic examination of preparations of different materials, both native and Gram-stained. Candida grows on many standard nutrient mediums, particularly well on Sabouraud agar. Typical yeast colonies are identified under the microscope and based on specific metabolic evidence.

Detection of Candida-specific antigens in serum (e.g., free mannan) is possible using an agglutination reaction with latex particles to which monoclonal antibodies are bound. Various methods are used to identify antibodies in deep candidiasis (agglutination, gel precipitation, enzymatic immunoassays, immunoelectrophoresis).

Therapy. Nystatin and azoles can be used in topical therapy. In cases of deep

6candidiasis, amphotericin B is still the agent of choice, often administered together with 5-fluorocytosine. Echinocandins (e.g., caspofungin) can be used in severe oropharyngeal and esophageal candidiasis.

Epidemiology and prevention. Candida infections are, with the exception of candidiasis in newborn children, endogenous infections.

Aspergillus (Aspergillosis)

Aspergilloses are most frequently caused by Aspergillus fumigatus and A. flavus. A. niger, A. nidulans, and A. terreus are found less often. Aspergilli are ubiquitous in nature. They are found in large numbers on rotting plants.

Morphology and culture. Aspergillus is recognized in tissue preparations, exudates and sputum by the filamentous, septate hyphae, which are approximately 3–4 lm wide with Y-shaped branchings (Fig. 6.4).

Aspergillus grows rapidly, in mycelial form, on many of the mediums commonly used in clinical microbiology. Sabouraud agar is suitable for selective culturing.

Pathogenesis and clinical pictures. The main portal of entry for this pathogen is the bronchial system, but the organism can also invade the body through injuries in the skin or mucosa. The following localizations are known for aspergilloses: